A known hollow shaft reduction gear is shown in FIG. 10, and has a hollow output shaft, and a driven shaft inserted in the output shaft so as to be coupled with the output shaft.
As shown, the hollow output shaft B is extended through and supported for rotation in bearings C on a gear case A. A worm wheel E is mounted on the output shaft B and in engagement with a worm D connected to a drive means (not shown). The output shaft B is driven for rotation at an output rotating speed lower than the input rotating speed through the worm D. A plate F is mounted on the output shaft B in the gear case A to confront the worm wheel E so as to rotate together with the output shaft B, but so as to be axially slidable on the shaft B.
A hemispherical recess G is formed in a surface of the worm wheel E facing the plate F, a hemispherical recess H is formed in a surface of the plate F so as to correspond with the hemispherical recess G, and a ball I is interposed between the hemispherical recesses G and H. The plate F is biased continuously toward the worm wheel E by a spring J so that the ball I is held between the respective hemispherical recesses G and H of the worm wheel E and the plate F. When the worm wheel E is rotated through the worm D by the drive means, a driving torque is transmitted through the ball I and the plate F to the output shaft B.
When the driven shaft (not shown) coupled with the output shaft B is overloaded, the plate F is moved away from the worm wheel E against the biasing force of the spring J. Consequently, the ball I moves out of the hemispherical recess of the plate F or the worm wheel E and the worm wheel E and the plate F slip relative to each other.
In the known hollow shaft reduction gear thus constructed, it is difficult to adjust transmission torque because an overload protecting device is placed in the gear case, and the gear case needs to be disassembled for torque adjustment. When the overload protecting device needs repair or the parts of the overload protecting devices need to be replaced, the gear case must be disassembled, which requires difficult work.